Shunt systems for directing body fluid from one region to another are known in the medical field. One application for such a shunt system is in the treatment of hydrocephalus, a condition where cerebrospinal fluid collects in the ventricles of the brain of a patient. Cerebrospinal fluid is produced by the ventricular system and is normally absorbed by the venous system. However, if the cerebrospinal fluid is not absorbed, the volume of cerebrospinal fluid increases thereby elevating the patient's intracranial pressure. This excess cerebrospinal fluid can result in abnormally high epidural and intradural pressures. Left untreated, hydrocephalus can result in serious medical conditions, including subdural hematoma, compression of the brain tissue, and impaired blood flow.
To treat patients with hydrocephalus, shunt systems have been used to remove the excess cerebrospinal fluid and to discharge the fluid to another part of the patient's body, such as the right atrium or peritoneal cavity. By draining the excess fluid, the elevated intracranial pressure is relieved. Generally, these fluid shunt systems include a valve mechanism for controlling or regulating the flow rate of fluid through the shunt system. The shunt systems often include a brain ventricular catheter in fluid communication with the valve mechanism. The ventricular catheter is inserted into a ventricle of the brain and a peritoneal catheter, which is also in fluid communication with the valve mechanism, is inserted into the peritoneal cavity of the patient for discharging the excess cerebrospinal fluid. The valve mechanisms of these shunt systems typically operate to permit fluid flow only once the fluid pressure reaches a certain threshold level. The fluid flow rate is proportional to the pressure at the valve mechanism. Thus, for a pressure slightly greater than the threshold or opening pressure, the flow rate is relatively low. As the pressure increases the flow rate through the shunt system concomitantly increases. At pressures significantly greater than the threshold pressure, a maximum flow rate for the system is reached. Fluid flow normally continues until the intracranial pressure has been reduced to a level less than the threshold pressure, subject to any hysteresis of the device.
The threshold or opening pressure that allows fluid flow through a shunt system must often be adjusted. For example, a surgeon may initially select a relatively low opening pressure to trigger fluid flow. Over time, the initial opening pressure may not be ideal. For example, it could lead to excess fluid flow, creating an undesirable overdrainage condition in which too much fluid is drained from the ventricle. Such a situation may give rise to a need to increase the opening pressure to produce a fluid flow rate that is balanced to avoid both excessive intracranial pressure and overdrainage conditions.
Because physiologies will vary over time and from one individual to another, some valve systems have been designed to be adjustable without requiring invasive procedures. These adjustable valves allow the clinician to customize the implanted valve mechanism's opening pressure for a particular patient, without the need to surgically remove the implanted shunt system, adjust the valve mechanism, and then surgically implant the shunt system again. Such an adjustable valve system is described in, for example, U.S. Pat. Nos. 4,595,390, 4,615,691, 4772,257, and 5,928,182, all of which are hereby incorporated by reference. Commonly referred to as the Hakim programmable valve, the Hakim valve described in these patents is a differential pressure valve with very precise opening pressures determined by the force exerted on a ruby ball in a ruby seat. The pressure at which the valve opens can be adjusted non-invasively by the clinician by means of an externally applied rotating magnetic field. The valve opening pressure is adjusted by varying the spring tension exerted on the ruby ball. Applying an external magnetic field to energize the soft magnet stator components of the valve initiates the adjustment cycle. The magnetic field causes the rotor to rotate about a central axis. As the stator polarity is cycled, the rotor (cam) moves to different positions to align with the stator. These components perform together as a stepping motor. The spring rides along the cam; as the cam rotates clockwise or counter-clockwise, the spring tension increases or decreases, respectively. Other exemplary types of adjustable shunt valves are described in U.S. Pat. Nos. 5,637,038 and 5,643,194.
Current practice recommends an x-ray be taken after each valve adjustment to verify the new setting. The use of additional energy means to conventionally determine valve position, however, can often lead to undesirable complications. For instance, when magnetic fields are used for verifying valve position, metallic equipment within the clinical environment may interfere with the accuracy of information obtained through the use of these magnetic forces, leading to inaccurate readings.
There is thus a need for a non-invasive means of accurately verifying the position of an implanted adjustable valve within a patient so that repeated exposure of the patient to radiation energy is reduced or eliminated. Also desirable is a valve position verification device that is small, easy to use, and preferably portable. Preferably, the device can also monitor various valve functions without the necessity for additional energy means.